Why do mature mammalian red blood cells (RBCs) and sieve tube cells of vascular plants present a unique conceptual challenge to the strict definition of the classical Cell Theory?

Options:

• A: They do not originate from pre-existing cells but are formed de novo.
• B: They lack a nucleus at maturity, rendering them incapable of division, yet they perform vital metabolic functions.
• C: They are completely devoid of cytoplasm and cell membranes.
• D: They are multicellular structures acting as a single cell.

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